Alkaline cells are well known in the art and generally employ a zinc anode, a manganese dioxide cathode and an aqueous solution of potassium hydroxide for the electrolyte. These cells are readily available commercially for industrial and home applications. In conventional alkaline cells, the manganese dioxide is shielded from atmosphere (air) through seal means. To achieve maximum power output in a cell requires maximum amounts of the active components to be assembled in the cells.
Cathode containers and collectors for alkaline cells have used steel as the material for the container. The surface iron of the container oxidizes to a protective coating in high concentration aqueous potassium hydroxide electrolyte. However, this iron oxide coating is not a good electron conductor, and the thicker it gets the higher the resistance. Nickel is even worse than iron, in that the oxide is an even poorer electron conductor, however iron-nickel alloys are better than iron or nickel alone, and cobalt with nickel and iron is better than iron or iron-nickel. All the metal coatings show an increase in contact resistance with time as the oxide thickness builds up.
It has long been recognized that the contact between the container and the positive electrode is an important factor in cell design. In the prior art, experiments with container coatings were performed and it was found that a favored container coating was a manganese nitrate, EMD, graphite and water mixture, baked at 350.degree. C. to convert the nitrate to MnO.sub.2. A preferred conductive coating for containers today seems to be a graphite paint. This is a suspension of a binder and graphite powder in a liquid carrier. The paint is sprayed, swabbed, brushed, etc., onto the surface to be coated. When the liquid carrier evaporates, graphite particles, adhered to each other and to the metal surface by the binder, remain on the surface. The volume percent of the binder, which is usually an insulator, should be just adequate to bind the graphite so that it will not electrically isolate the graphite particles.
Japanese Patent Application No. 8-264193 discloses an alkaline manganese battery using a positive electrode containing manganese dioxide and a carbon material tightly contained in a positive electrode container made of metallic material. A conductor film, containing a material selected from the group comprising an oxide of Ag or Ag and Ni and a binder, is sprayed on the contacting surfaces of the positive electrode container and the positive electrode.
Another problem with graphite coatings is that they are easily scraped off by engagement with the positive electrode. Japanese Patent Application No. 8-264193 proposes to solve this problem by spraying a solution of a silver compound/binder onto the container, but this coating is also easily scraped or peeled off. Inherently, any coating added to the container surface before the positive electrode is inserted into or formed within the container is subject to damage from abrasion by the electrode material. The process described in this reference also requires an additional step of spraying the interior surface of the container, and the sprayed-on material must then be dried before the positive electrode can be inserted or formed within the container. The amounts of silver required by the process of this reference are large because of the method of application. These large amounts of silver oxide are soluble in the electrolyte. Some silver ions may migrate through the separator to the anode and react with the active anode material to generate hydrogen gas. Silver ions may also migrate to the separator and cause short circuits during discharge of the cell.
The applicant has also found that sprayed-on conductive coatings, such as graphite or silver with a binder, are prone to delaminate from the can, resulting in drastic performance reductions. The delamination does not need to be pronounced to produce these performance reductions. Small separation distances between the coating and the container allow capillary convection of electrolyte to produce a sandwich-like structure incapable of effective electron transfer. The sandwich structure (delaminated coating on one side, electrolyte in the center and the can surface on the other side) acts as a very effective electrical insulator, much like a double pane window acts as an effective heat insulator. The electrolyte stagnant between the coating and the can surface prevents electron transfer between these two surfaces.
It is an object of the present invention to provide a process for assembling an electrochemical cell with a positive electrode container in which the surface of the wall of the container that contacts the positive electrode has a deposited layer of silver.
It is another object of the present invention to provide a process for assembling an alkaline manganese dioxide cell that is cost effective to produce and easy to assemble.
It is another object of the present invention to provide an electrochemical cell that has a positive container with a wall that contacts the positive electrode having a silver layer formed in situ between the wall and the positive electrode.
It is another object of the present invention to provide an alkaline manganese dioxide cell that has a silver layer formed in situ on the wall of the positive container that contains the positive electrode.
It is another object of the present invention to provide a conductive surface for the positive electrode container after insertion or formation of the positive electrode.
It is another object of the present invention to provide a conductive surface for the positive electrode container which contains a minimum amount of silver.
It is another object of the present invention to provide a process of preparing the surface of a positive electrode container without the addition of more process steps.
It is another object of the present invention to provide a conductive surface for the positive electrode container which allows for the elimination of any cladding of the positive electrode container.
The above and further objects will become apparent upon consideration of the following description and drawing thereof.